This invention relates to an auto-tensioner used to keep constant the tension of mainly a timing chain or a timing belt for driving engine camshafts.
Ordinarily, the rotation of a crankshaft of an engine is transmitted to camshafts through a timing chain or a timing belt (in the following description, only the timing chain is discussed) to rotate the camshafts, thereby selectively opening and closing the valves for combustion chambers.
An auto-tensioner is ordinarily used to keep constant the tension of the chain. One known auto-tensioner includes a cylinder having an open front end, a plunger axially slidably inserted in the cylinder and having a closed front end and an open rear end which is located inside the cylinder, and a screw rod having an external thread formed on the outer periphery thereof and in threaded engagement with an internal thread formed on the inner periphery of the plunger. The screw rod has a rear end protruding from the open rear end of the plunger and in abutment with a seat member provided in the cylinder. The plunger is biased outwardly of the cylinder by a return spring having one end thereof supported by the screw rod, thereby pressing the front end of the plunger, which protrudes from the cylinder, against the chain or belt (see JP Patent Publication 2007-32603A).
While the engine is running, the plunger of this auto-tensioner moves back and forth within the range of the clearance between the internal and external threads, thereby absorbing vibrations of the chain. When the tension of the chain changes, the plunger slowly turns while moving back and force within the range of the clearance between the internal and external threads, and moves to a position where the biasing force of the return spring balances with the tension of the chain.
When the engine is stopped, according to the positions of the cams when they stop, the tension of the chain may be higher than the force of the return spring. But while the engine is at a stop, since the chain does not vibrate, the plunger does not move. Thus, when the engine is restarted, the chain is less like to slacken, so that the engine starts smoothly.
But since the moving speed of the plunger of this auto-tensioner is low when the plunger is pushed into the cylinder beyond the range of the clearance between the internal and external threads, if the amplitude of vibration of the chain is large due to a large change in torque of the crankshaft, a large change in rotational resistance of the camshafts or resonance of the engine during one combustion cycle, it may be impossible to absorb vibrations of the chain, thus resulting in over-tensioning of the chain.
Also, since the moving speed of the plunger of this auto-tensioner is low when the tension of the chain changes, if the travel path of the chain changes markedly due to thermal expansion of the engine block, it may be impossible to absorb such a change in the travel path of the chain, thus resulting in over-tensioning of the chain.
An object of this invention is to provide an auto-tensioner which can minimize the possibility of over-tensioning of the chain even if the chain vibrates markedly or the travel path of the chain changes markedly.